Charging
Why does my image look different at different speeds?
A distorted image at a slow scanning speed is usually a sign of charging.
Charging manifests in a few forms:
Drift: the sample appears to move continuously in one direction when the stage is stationary.
High Contrast: for example, a very bright feature surrounded by an annular dark ring.
Pulsing or Fluctuating Brightness: the same feature appears bright in one scan and dark in the next, or may actively fluctuate if scanning at a fast speed.
Streaks: bright or dark streaks across an entire image, especially if they change with each scan.
Blooms: growing and changing "features" on the edge of cross section samples
How can I prevent charging?
Charging is usually the result of electron buildup beneath the sample surface. This buildup slows the incoming beam, which leads to higher secondary electron emissions. The increased emission may clear the charge buildup, but it returns on the next scan.
Charging is influenced by the electron dose (accelerating voltage and current) and sample conductivity. This may be the conductivity of the material itself, the morphology of the sample, (e.g. fiber bundles or stacks of particles are less conductive because the grounding path is interrupted,) or the quality of the conductive path between the sample surface and the stub.
In most cases, decreasing the accelerating voltage and/or beam current reduces charging. Some samples will charge regardless and must be imaged at fast speeds (typically with frame averaging or integration) or in low vacuum. In rare cases, a higher accelerating voltage will reduce charging - for example, for very thin films.